1. Field of the Invention
The present invention relates to a multicylinder internal combustion engine, and more particularly to a multicylinder internal combustion engine in which an exhaust gas sensor for determining the exhaust air-fuel ratio is provided in an exhaust system.
2. Description of the Related Art
As one of exhaust emission control devices which purify exhaust gas in an internal combustion engine, catalysts are provided in an exhaust passage of the engine. The air-fuel ratio of exhaust gas flowing into the catalysts must be controlled to an approximately stoichiometric ratio so that the catalysts can offer satisfactory performance. A multicylinder internal combustion engine adapted to attain such an object has been disclosed in Unexamined Japanese Patent Publication No. 11-280458, for example. In this multicylinder internal combustion engine, exhaust gas sensors such as an O2 sensor are disposed in the vicinity of the inlets of the catalysts so that the air-fuel ratio of the internal combustion engine can be controlled based on the exhaust air-fuel ratio of determined by the exhaust gas sensors. It should be noted that the catalysts are provided in respective one of a pair of exhaust passages located upstream of a junction thereof, and a common exhaust gas sensor is provided in a communication passage that brings the exhaust passages in the vicinity of the inlets of the catalysts into communication with each other.
In the case where the exhaust gas sensors are provided in the vicinity of the inlets of the catalysts as is the case with the above internal combustion engine, an exhaust system with a certain degree of volume exists upstream of the exhaust gas sensors. Thus, determination of the air-fuel ratio by the exhaust gas sensors has a delay corresponding to a delay in exhaust gas transmission which occurs due to the volume of the exhaust system. Also, in the case where the air-fuel ratio of the internal combustion engine is controlled based on the exhaust air-fuel ratio determined by the exhaust gas sensors, fluctuation in air-fuel ratio (self-excited fluctuation in air-fuel ratio) necessarily occurs due to the delay in the determination of exhaust air-fuel ratio. If there is a long delay in the determination of exhaust air-fuel ratio, the amplitude of fluctuation in the air-fuel ratio of air-fuel mixture supplied to the internal combustion engine is increased. As a result, fuel economy is likely to deteriorate due to an increase in the amplitude of fluctuation to the rich side, and also, combustion is likely to deteriorate due to an increase in the amplitude of fluctuation to the lean side.
The delay in the determination of exhaust air-fuel ratio can be eliminated by, for example, attaching exhaust gas sensors to respective exhaust ports of the internal combustion engine to reduce the volume of the exhaust system located upstream of the exhaust gas sensors. In this case, however, it is necessary to provide the exhaust gas sensors for the respective exhaust ports if the internal combustion engine is a multicylinder type, and hence another problem that manufacturing costs increase arises.